This invention relates to magnetostrictive actuators, and more particularly to automotive actuators, such as fuel injectors and electronic valve timing actuators that operate based on the principle of magnetostrictive transduction. Still more particularly, this invention relates to magnetostrictive alloy compositions formulated to provide improved temperature response for automotive applications without the need for complex electronic controls and a method and apparatus using the same. Still more particularly, this invention relates to the application of optimal or near-optimal prestress forces to the magnetostrictive alloy compositions to improve the magnetostrictive response.
A conventional method of actuating fuel injectors is by use of an electromechanical solenoid arrangement. The solenoid is typically an insulated conducting wire wound to form a tight helical coil. When current passes through the wire, a magnetic field is generated within the coil in a direction parallel to the axis of the coil. The resulting magnetic field exerts a force on a moveable ferromagnetic armature located within the coil, thereby causing the armature to move a needle valve into an open position in opposition to a force generated by a return spring. The force exerted on the armature is proportional to the strength of the magnetic field; the strength of the magnetic field depends on the number of turns of the coil and the amount of current passing through the coil.
In the conventional fuel injector, the point at which the armature, and therefore the needle, begins to move varies primarily with the spring preload holding the injector closed, the friction and inertia of the needle, fuel pressure, eddy currents in the magnetic materials, and the magnetic characteristics of the design, e.g., the ability to direct flux into the working gap. Generally, the armature will not move until the magnetic force builds to a level high enough to overcome the opposing forces. Likewise, the needle will not return to a closed position until the magnetic force decays to a low enough level for the closing spring to overcome the fuel flow pressure and needle inertia. In a conventional injector design, once the needle begins opening or closing, it may continue to accelerate until it impacts with its respective end-stop, creating wear in the needle valve seat, needle bounce, and unwanted vibrations and noise problems.
Magnetostrictive fuel injectors are believed to solve many of these problems by providing a fuel injector actuation method that provides reduced noise, longer seat life, elimination of bounce, and full actuator force applied during the entire armature stroke. The term xe2x80x9cmagnetostrictionxe2x80x9d literally means magnetic contraction, but is generally understood to encompass the following similar effects associated with ferromagnetic materials: the Guillemin Effect, which is the tendency of a bent ferromagnetic rod to straighten in a longitudinal magnetic field; the Wiedemann Effect, which is the twisting of a rod carrying an electric current when placed in a magnetic field; the Joule Effect, which is a gradual increasing of length of a ferromagnetic rod when subjected to a gradual increasing longitudinal magnetic field; and the Villari Effect, which is a change of magnetic induction in the presence of a longitudinal magnetic field (Inverse Joule Effect).
The dimensional changes that occur when a ferromagnetic material is placed in a magnetic field are normally considered undesirable effects because of the need for dimensional stability in precision electromagnetic devices. Therefore, manufacturers of ferromagnetic alloys often formulate their alloys to exhibit very low magnetostriction effects. All ferromagnetic materials exhibit magnetic characteristics because of their ability to align magnetic domains. As shown in FIG. 1, strongly magnetostrictive materials characteristically have domains that are longer in the direction of their polarization (North/South) and narrower in a direction perpendicular to their polarization, thus allowing the domains to change the major dimensions of the ferromagnetic material when the domains rotate.
For example, the magnetostrictive alloy Terfenol-D (e.g., Tb0.3Dy0.7Fe1.9), is capable of approximately 10 um displacements for every 1 cm of length exposed to an approximately 500 Oersted magnetizing field. The general equation for magnetizing force, H, in Ampere-Turns per meter (1 Oersted=79.6 AT/m) is: H=IN/L, where I=Amperes of current; N=number of turns; and L=path length.
Terfenol-D is often referred to as a xe2x80x9csmart materialxe2x80x9d because of its ability to respond to its environment and exhibit giant magnetostrictive properties. However, in order to realize the full potential of a magnetostrictive actuator, it would be desirable for the amount of magnetostrictive elongation to be independent of temperature over the range of temperatures typically encountered in automotive applications. The conventional solution to temperature compensation of magnetostrictive materials requires the use of complex electronic controls to control the current in the coil to compensate for the non-linear temperature response in the magnetostrictive material. The present invention eliminates the need for such electronic compensation, thereby greatly reducing the complexity of the control system required to control the magnetostrictive actuator.
A magnetostrictively actuated fuel injector is provided. The fuel injector includes a body having an inlet port, an outlet port and a fuel passageway extending from the inlet port to the outlet port. A metering element is disposed proximate the outlet port. A magnetostrictive element is in operative contact with the metering element. The composition of the magnetostrictive element provides a substantially linear temperature response over the range of temperatures from approximately xe2x88x9240xc2x0 C. to +150xc2x0 C. A coil is proximate the magnetostrictive element such that, upon excitation of the coil, magnetic flux generated by the coil causes the magnetostrictive element to change length and actuate the metering element.
A compound for use in a magnetostrictive valve is also provided. The compound consists essentially of the formula: TbxDy1-xFey, wherein x ranges from about 0.31 to about 0.33 and y ranges from about 1.8 to about 2.2.
A method of using a compound of the formula TbxDy1-xFey to form a fuel injector, wherein x ranges from about 0.31 to about 0.33 and y ranges from about 1.8 to about 2.2, is also provided. The method includes forming the compound into a magnetostrictive element for use as a magnetostrictive actuator in a fuel injector